Evaluation of the potential of FDM, MJF, and SLS printing technologies to manufacture reproducible and high-cycle fatigue resistant polymer springs

Abstract

Elastic structures from polymer are a promising alternative to conventional metal springs under challenging conditions like in magnet resonance imaging (MRI) environments, where ferromagnetic materials cannot be used. The suitability of additive manufacturing to manufacture polymeric springs was assessed within the paper with a dedicated focus on stiffness reproducibility and fatigue behavior. Multi-Jet Fusion (MJF), Selective Laser Sintering (SLS), and Fused Deposition Modeling (FDM) were evaluated as manufacturing technologies. Three spring designs were conceived based on a heuristic approach, taking into account the constraint of anisotropic material behavior. MJF and SLS were used to print all three designs, respectively. Use of FDM was limited to print one design, the others were not appropriate for FDM due to high complexity. The anisotropy in mechanical characteristics of SLS and MJF printing technologies was assessed to estimate its possible influence on spring stiffness. MJF-printed tensile specimens show more anisotropic material behavior compared to SLS-printed ones. Overall suitability of FDM to print springs was shown to be limited due to design constraints and manufacturing limitations like warping and in-plane delaminations between deposited polymer strands, as well as very limited applicable cycles during fatigue testing. MJF-printed springs showed higher variability in geometric dimensions compared to SLS. Slight variances in geometric dimensions were shown to crucially influence spring stiffness, thus lowering the reliability of the MJF technology for reproducible springs. Fatigue life of either SLS or MJF samples was shown to be appropriate as all springs survived 100,000 load cycles with moderate loss of reaction force below 16% for MJF- and below 10% for SLS-printed springs. SLS was shown to be the most promising of the three evaluated printing technologies for small-scale series manufacturing of springs considering reproducibility as well as fatigue behavior.